1. Field of the Invention
The present invention relates to a light-receiving module which receives light signals in optical fiber transmissions. In particular, the invention achieves an improvement of inexpensive resin-molded package elements, and it is therefore an object of the invention to provide a resin-molded package having a structure by which the light receiving part is protected from external noise.
2. Prior Art
In a light-receiving module of a prior art, a receiving photodiode (PD module: as shown in FIG. 1) was housed and shielded in an expensive metallic casing.
A description is given of a light-receiving module of a prior art. In a PD module 15 in FIG. 1, a PD chip 17 is fixed at the middle of the upper surface of a metallic disk stem 16. A metallic cap 18 is fixed on the stem 16. A metallic cylindrical lens holder 19 is welded to the stem 16 so that it encloses a cap 18. Also, a conical type ferrule holder 20 is fixed on the lens holder 19.
A metallic ferrule 22 is fixed at the tip end of an optical fiber 21. The ferrule 22 is inserted into a top hole of the ferrule holder 20, and fixed therein. The tip ends of the optical fiber and ferrule are polished to be slant so that no reflected returning light enters an LD.
The PD module 15 is accommodated in a metallic casing consisting of a metallic stem 16, metallic lens holder 19 and metallic ferrule holder 20. The light receiving part is likely to be influenced by peripheral noise since the impedance thereof is high and the signal current thereof is small. However, since the light receiving part is covered by a metallic can, it is possible to be electrically shielded from peripheral noise.
An amplification circuit which amplifies the light current connects the PD module. This is also accommodated in a metallic casing and is designed to be resistant to noise. FIG. 2 shows an example of such a light receiving circuit. A wiring pattern is depicted on an epoxy circuit substrate 30. The lead pins of the PD 15 housed in a metallic can in FIG. 1 is inserted into a wiring pattern 32 of the substrate 30 and soldered therein. An amplification circuit 31 connected the wiring pattern 32 is also an element covered by a metallic casing. The metallic casing is connected to a ground terminal on the circuit substrate. A wire 33 extending from an amplification circuit 31 enclosed by a metallic casing is connected to an output circuit and a power source.
Since, in the light receiving part, the signal current is low and the amplification is high, the light receiving part is apt to receive noise. As regards the prior art light-receiving module, the PD module, drive circuit, and amplification circuit are individually housed in separate casings. Therefore, even though the light-receiving module receives light in an external surrounding where noise exists, there is almost no influence where external noise reaches and is mixed with weak receiving signals.
Where the PD module in FIG. 1 is used, there is no invasion of external noise into the PD (light receiving part) circuit. However, there are a considerable number of elements and parts. It is necessary for wires to be printed on a printed substrate 30. The amplification circuit 31 enclosed by a metallic casing is soldered thereto, and a pin of the PD module housed in a metallic can in FIG. 1 is further soldered thereto. As described above, amplification circuit elements housed in independent metallic cans are required, and printed substrates are also required. That is, the number of components is increased, thereby causing an increase in the production cost.
There are still further problems even in the module in FIG. 1, one of which is that the stem face and the optical fiber direction are vertical, and the other of which is that of light propagating in space over a considerable distance in a package. Therefore, a lens 26 is also required. The PD module is large-sized, and the light-receiving module in which an amplification part is incorporated will become large-sized as shown in FIG. 2. The size of the light-receiving module will inevitably increase.
Further, since expensive metallic packages are used for PD and circuits such as an amplification circuit, production cost is increased. Insofar that an expensive and large-sized light-receiving module is used, light transmission cannot extend to general household use. It is essential that light-receiving modules are small-sized and inexpensive in order to allow extensive use of light transmission to include general household use. If individual components are housed in expensive metallic packages, low cost construction is difficult. Further, in view of making them small-sized, it is preferable that the PD and amplifier (AMP) are not separate from each other.
Recently, an attempt has been made to achieve a small-sized and inexpensive product by mounting a waveguide (or optical fiber), PD chip and AMP chip on one substrate. A PD and amplifier are mounted on one substrate, and light is caused to propagate in parallel on the substrate. Since a waveguide or an optical fiber, and a PD and an amplifier are mounted on a planar substrate, this is called xe2x80x9cplanar mountingxe2x80x9d. The example is illustrated in FIG. 3 (Plan view) and FIG. 4 (Cross-sectional view). A light waveguide 44 is formed on an Si bench 42, and a PD 47 is placed on the termination end, and an amplifier (AMP) 48 chip is provided on the same substrate 42.
The termination end of an optical fiber 49 is connected to the end of the light waveguide 44. Light propagating through the optical fiber 49 is input from the light waveguide 44 to the waveguide-type light-receiving layer of the PD 47. That is, the light is converted to photocurrents. The photocurrents are immediately amplified by the amplifier 48. Since light signals of the PD are directly amplified beforehand without routing in any unnecessary wiring, the light signals have high resistance against noise.
Since the waveguide or optical fiber, PD and amplifier are placed on one substrate, the optical receiving module can be small-sized and lightweight. Since the chips themselves are small, they can be disposed adjacent to each other, whereby the elements can be made further small-sized. The number of packages (casings) is also reduced one, and the cost thereof can be decreased. If a ceramic package is employed with a reduced number of packages, the hermeticity can be improved. However, the cost is not reduced. One of the most inexpensive packages is a commonly available resin mold. Therefore, it is highly advantageous to employ a resin mold package in order to reduce the production cost.
In such a pattern where the receiving signal light of the PD is intense (that is, a short-range transmission), it will cause almost no problem. However, as the receiving signals become weak due to long-range transmission, the problem of noise occurs. If only a resin mold is employed, there is no shielding effect against external noise. Since the input impedance is high with a high amplification ratio noise is likely to be detected despite the light receiving part consisting of a PD and an amplifier. That is, the light receiving part is of a structure that is liable to be affected by noise, and sufficient reception cannot be obtained with respect to weak signals since noise is not shielded. It is therefore an object of the invention to provide an inexpensive and small-sized light-receiving module which resolves such problems, and protect the light receiving part thereof from external noise.
A light-receiving module according to the invention has an optical fiber or a light waveguide, a light receiving element PD and an amplifier (AMP), wherein the light receiving part is covered by a conductive material having openings through which resin can pass, for example, a metallic plate, metallic meshed member, metallic porous plate having a plurality of pores, and the light receiving part, metallic meshed member and metallic porous plate are covered and molded by resin, and all of these are integrated as one unit.
In the invention, the light receiving part is covered by a conductive material having openings through which resin passes, for example, folded metallic plate, porous metallic plate, and metallic meshed member, thereby electrically shielding the light receiving part from the outside. That is, the light receiving part is electrically shielded from external intensive electric waves by the open folded metallic plate, metallic meshed member, or porous metallic plate. The light receiving part is electrically shielded by the open folded metallic plate, metallic meshed member, or porous metallic plate because the light receiving part is densely covered by a molding material invading the interior through the openings (pores). Although mere shielding may be made sufficient by putting a metallic cap over the light receiving part, if the light receiving part is shielded by a metallic cap, no resin can be inserted into the light receiving part in the process of molding, wherein the light receiving part cannot be molded by resin. In the invention, the light receiving part is covered by a metallic plate having openings, wherein fluid resin is inserted from the outside into the light receiving part through the openings and is solidified therein. That is, fluid resin is supplied, and the light receiving part is electrically shielded by a metallic plate having openings, a meshed member, or porous metallic plate to mold the light receiving part. While large numbers of openings and openings large in diameter facilitate the insertion of resin into the interior, oversized pores reduce the ability to shield electric waves. Therefore, the number of pores, and the size thereof are adequately determined by the viscosity of fluid resin and the noise level. While it is preferable that the light receiving part is covered by a metallic part in the case of a ceramic package, the ceramic package is costly. The elements of the invention are packaged by a mold type package specifically to reduce the production cost. Also, in order to prevent external noise from entering, it is strongly recommended that the light receiving part be shielded by placing the PD and AMP (amplifier) as close together as possible. The mold type enables an equivalent outer form of the package to the prior art packages to be made while shielding the PD and AMP in the closest position.
Where the PD and AMP (amplifier) are integrated in the light-receiving module as shown in FIGS. 3 and 4, they can be made smaller-sized than separated PD and AMP as in FIGS. 1 and 2. For example, even though the PD and AMP are integrated and constructed as shown in FIGS. 3 and 4, it is costly if a ceramic package is used. However, since a resin-molded package is employed in the invention, without the use of any ceramic package, the cost of the package is reduced. Further enhanced from the states shown in FIGS. 3 and 4, external noise is prevented from permeating by covering the light receiving part with a metallic plate having openings, a metallic meshed member or porous metallic plate. There are two modes in coupling with optical fibers, one of which is a waveguide type (a) in which a light waveguide is formed on a substrate, and the other a V-grooved type (b) in which an optical fiber ribbon is fixed in a V-shaped groove. The invention is applicable to any one of these types.
(a) Waveguide type (FIGS. 5 and 6) . . . A PD chip 47 and an amplifier chip 48 are provided on a substrate 42 in parallel. A light waveguide 44 is provided on the substrate 42. The end face of the optical fiber 49 is connected to the starting end of the light waveguide 44, and the PD 47 is coupled to the termination end of the light waveguide 44. The PD 47 and amplifier 48 are covered by a porous metallic plate (herein, a meshed member 53) in order to prevent from permeation of external noise.
(b) V-grooved type (FIGS. 7 and 8) . . . A PD chip 47 and an amplifier chip 48 are provided on a substrate 42 in parallel. A V-shaped groove 45 is provided on the substrate 42. The ribbon 46 which is obtained by unsheathing the optical fiber 49 is fixed in the V-shaped groove 46. The PD 47 is coupled to the termination end of the ribbon 46. The PD 47 and AMP 48 are covered y a porous metallic plate (herein, a meshed member) to prevent from permeation of external noise.
The present invention may be applicable to either (a) or (b). In the invention, an optical fiber or a light waveguide, PD and AMP are fixed on the same substrate. And further, they are resin-molded to cover the light receiving part by a porous metallic plate. Therefore, the invention can provide a small-sized and inexpensive light-receiving module having high reliability.
The feature of the invention resides is that the light receiving part is electrically shielded by a conductive body having open portions and is resin-molded. The feature of a light-receiving module according to the invention is that the light receiving part is covered by a metallic meshed member, a porous metallic plate, or a metallic plate, etc., whereby external electric waves can be divert d so as not to enter the light receiving part as noise. Since the noise level is low, the reception is heightened. Further, the light-receiving module being planar mounting type facilitates its production. Also, small-size can be achieved since a planar mounting type optical fiber and light receiving part are secured the same substrate. Still further, since a light-receiving module according to the invention is of a resin-mold type, production cost can be reduced. In line with prevailing optical transmission networks, a low cost module Is essential. That is, by the invention, a noise-suppressed and small-sized light-receiving module having high performance can be industrially mass-produced at a low production cost.